Question

We breathe through our nostrils and not through our mouth.why?

Answer 1

Because when we breathe through nostril the air we breath get filtered out through the nostril hairs and dust particle do not enter inside as they get stick to the nostril hair and air that enters the lungs becomes humid and dust free.

Answer 2

the fact behind this is that the air passing through the nostrils is filtered by the fine hairs in the nasal chambers that  line the  passage so that air going into the body is free of dust and other impurities. on the other hand our mouth doesn't have such features. so we breathe through our nostrils and not through our mouth